Related fields include semiconductor devices and their fabrication; in particular, thin-film components of resistive-switching non-volatile memory (ReRAM).
Nonvolatile memory elements are used in computers and other devices requiring persistent data storage (e.g., cameras, music players). Some traditional nonvolatile memory technologies (e.g., EEPROM, NAND flash) have proven difficult to scale down to smaller or higher-density configurations. Therefore, a need has developed for alternative nonvolatile memory technologies that can be scaled down successfully in terms of performance, reliability, and cost.
In resistive-switching-based nonvolatile memory, each individual cell includes a variable resistor. It can be put into either of at least two states (e.g., a low-resistance state and a high-resistance state), where it will stay until receiving a signal input that changes it to the other state (a “write,” “set,” or “reset” signal). The resistive state of the variable resistor corresponds to a bit value; e.g., the low-resistance state may represent logic “1” and the high-resistance state may represent logic “0”. The cell is written by applying a signal that changes the resistance of the variable resistor, and is read by sensing its present resistance without changing it.
Many ReRAM devices change resistance by creating and destroying, or lengthening and shortening, one or more conductive paths through a variable-resistance layer or stack while the bulk material remains substantially static (e.g., it does not change phase). The bulk material is often a highly insulating dielectric. The conductive paths (also known as “percolation paths”) are formed when an electric field organizes conductive or charged defects or impurities into a filament stretching from one interface to the other, with sufficient defect density that charge carriers can easily traverse the layer by tunneling from defect to defect. To return the variable resistor to the high-resistive state, it is often not necessary to destroy the entire filament, but only to introduce a gap too wide for tunneling somewhere along the filament's length. Some of the types of defects that have been used include metal clusters and oxygen (or nitrogen) vacancies.
Preferably, write and read operations should require as little power as possible, both to conserve energy and to avoid generating waste heat. Preferably, the different resistance states should be easily distinguishable by sensing with a low current. Repeatability and cell-to-cell consistency of the resistance values contribute to certainty in sensing the resistive state; variations from cycle to cycle or cell to cell put “error bars” around the resistance values and make them less distinguishable.
Preferably, the different resistance states should be stable over long periods (e.g., years). Therefore, there should only be significant movement of defects in the cell when a write signal is applied. If defects migrate in an uncontrolled manner between write signals, even very slowly, the written data will eventually be lost.
So far, meeting all these goals in a single design has been challenging. Therefore, a need exists for ReRAM cell designs and fabrication methods that enable low-power operation, reduce the potential for reading errors by improving cycle-to-cycle repeatability and cell-to-cell consistency, and preserve stable resistance states over long time periods.